1. Field of the Invention
The present invention relates to high-strength hydraulic cement compositions, and, more particularly, to high-strength hydraulic cement compositions comprising phosphate compounds and calcium silicates, and methods for making and using the same.
2. Related Art
Hydraulic cements have very long history, but many aspects of their chemistry have yet to be completely understood.
Thousands of years ago, Pozzolana, a volcanic ash from Mount Vesuvius, was mixed with limestone, to prepare a powder that hardened (“set”) when mixed water. This pozzolanic (natural) cement was later reproduced artificially, by heat treating of a mixture of limestone, clay, and bauxite at temperatures in excess of 1500 C. The resulting cement “clinker” was crushed and mixed with gypsum and other additives to result in ordinary Portland cement (OPC). The process is largely unchanged today, although minor changes in composition and various additives have been introduced to achieve particular properties. Current global production of OPC approaches 1 T for every person on earth each year, consuming vast amounts of resources and energy; thus any modification which would decrease the use of resources, while maintaining or improving the properties of cement, could have a significant impact on the environment and our civilization.
There are two main compounds in Portland cement: dicalcium silicate (C2S, also known as Alite) and tricalcium silicate (C3S, also known as Belite). Highly crystalline calcium hydroxide (Ca(OH)2) (referred to herein as CH) and amorphous calcium-silicate-hydrate (referred to herein as C—S—H) are formed in the hydration of these two principal components (C2S and C3S). The hydrated cement paste consists of approximately 70% C—S—H, 20% Ca(OH)2, 7% sulfoaluminate, and 3% secondary phases. Major problems caused by the calcium hydroxide, which is formed as a result of the setting reaction, are that CH is soluble in water and has low strength, which properties negatively affect the quality of concrete; as will be discussed below, the present invention decreases the final content of CH in the set cement, thus resulting in a significantly increased durability and strength.
General purpose Portland type cement (ASTM I) typically contains approximately 50% C3S, 25% C2S, 12% C3A (tricalcium aluminate 3CaO.Al2O3), 8% C4 AF (tetracalcium aluminoferrite 4CaO.Al2O3.Fe2O3), and 5% calcium oxide CaO. The total amount of calcium silicates (C3S and C2S) is approximately 75%, with the predominant silicate being C3S.
A number of investigators have reported achieving an improvement in the mechanical strength of Portland cement, by adding silica fume (SiO2, referred to as S) in order to decrease calcium hydroxide content in the hydrated cement (Mitchell, et al, “Interaction of silica fume with calcium hydroxide solutions and hydrated cement pastes”, Cement and Concrete Research (1998), 28(11), 1571-1584 and Persson “Seven-year study on the effect of silica fume in concrete” Advanced Cement Based Materials (1998), 7(3/4), 139-155). The mechanism of improvement depends on the silica fume reacting with calcium hydroxide to produce an amorphous C—S—H gel with a high density and low Ca/Si ratio. Therefore, no new phases are introduced to the set cement. This demonstrates that removal of CH from the set cement provides a substantial improvement in quality. The present invention discloses an alternative method of in-situ removal of CH from setting cement, by reactively precipitating calcium phosphates, in particular hydroxyapatite; this provides not only enhanced strength and degradation resistance but also enhanced biological properties.
Phosphate-based hydraulic structural cements ares well known (e.g., see Friedman et al “BoneSource hydroxyapatite cement: a novel biomaterial for craniofacial skeletal tissue engineering and reconstruction” Journal of Biomedical Materials Research (1998), 43(4), 428-432). However, these cements do not contain silicate material. Strength development and hardening of these cements during setting does not rely on hydration of calcium silicates, and does not involve precipitation of C—S—H gel and CH.
Ma et al (“Effect of phosphate additions on the hydration of Portland cement” Advances in Cement Research (1994), 6(21), 1-12) reported the effect of phosphate additions on the hydration process of Portland cement. The reaction products were amorphous, but hydrothermal treatment at 160° C. of ordinary Portland cement (OPC) modified by CaHPO4 allowed transformation of a poorly crystalline phosphate phase into hydroxyapatite. Generally, the presence of sodium and calcium phosphates resulted in improved flexural strengths. However, a number of disadvantages limit the usefulness of the process disclosed by Ma et al, such as the need for hydrothermal treatment to form hydroxyapatite and the need for high pressure (28 MPa) pressing in order to prepare samples having adequate strength. Also, the process described by Ma et al cannot be used to form a uniform composite structure, and the mechanical strength of the phosphate-modified samples was not significantly improved by comparison with Ordinary Portland cement
Chemically bonded ceramics (CBC), in the system CaO—SiO2—P2O5—H2O, were investigated by Hu et al (“Investigation of hydration phases in the system CaO—SiO2—P2O5—H2O” J. Mater. Res. 1988, 3(4) 772-78) and Sterinke et al (Development of chemically bonded ceramics in the system CaO—SiO2—P2O5—H2O” Cement and Concrete Res. 1991 (21)66-72). CBC powders were synthesized by a sol-gel process and then fired at a temperature of 700-1000 C for 2 hours. The components of the powders before hydration are calcium hydroxyapatite (major), calcium silicate hydrate, γ-2CaO.SiO2, amorphous calcium silicate, and amorphous calcium phosphate (Hu, et al, “Studies of strength mechanism in newly developed chemically bonded ceramics in the system CaO—SiO2—P2O5—H2O” Cement and Concrete Res. 1988 (18)103-108). However, the mechanical properties were not improved when the samples were hydrated at room temperature. Also, since the hydroxyapatite phase precipitated before hydration of the cement (i.e., it did not participate in the cement hydration), it did not reinforce the cement and thus did not contribute to its increased strength. In order to increase the mechanical strength of CBC, the samples were formed under high pressure (345 MPa) and were hydrated at high temperature.
Recently, Portland cement based materials (referred to as mineral trioxide aggregate, MTA) have been used for dental applications, such as endodontic dental treatment (Vargas et al., “A Comparison of the In vitro Retentive Strength of Glass -Ionomer Cement, Zinc-Phosphate Cement, and Mineral Trioxide Aggregate for the Retention of Prefabricated Posts in Bovine Incisors” J. Endodont. 30(11) 2004, 775-777). MTA consists primarily of tricalcium silicate, tricalcium oxide and silicate oxide. (Torabinejad et al. “Physical and chemical properties of a new root-end filling material”. J. Endodont 21(1995) 349-253). It is used in many surgical and non-surgical applications, and possesses the biocompatibility and sealing abilities requisite for a perforation material (Lee, et al, “Sealing ability of a mineral trioxide aggregate for repair of lateral root perforations” J Endod 1993; 19:541-4.). It can be used both as a nonabsorbable barrier and restorative material for repairing root perforations. Because it is hydrophilic and requires moisture to set, MTA is the barrier of choice when there is potential for moisture contamination, or when there are restrictions in technical access and visibility.
The physical and chemical properties of MTA have been tested and the initial pH on mixing was 10.2 rising to 12.5 after 3 hours. The MTA was demonstrated to be significantly less toxic than other root-end filing materials when freshly mixed, and toxicity was negligible when fully set at 24 h (Mitchell, et al, “Osteoblast biocompatibility of mineral trioxide aggregate” Biomaterials 20 (1999) 167-173) it also has good compressive strength after setting.
Torabinejad et al (U.S. Pat. No. 5,415,547 and U.S. Pat. No. 5,769,638) disclosed an improved method for filling and sealing tooth cavities using an MTA cement composition. The cement composition resembles Portland cement, and formed an effective seal against re-entrance of infectious organisms. However, the cement was gray in color, which is unsuitable for most dental applications. Moreover, although the MTA cement has been demonstrated to be non-toxic towards living tissue, it contains aluminum which is not well accepted by living tissue if released in ionic form. The hydration product of calcium aluminates are a mixture of calcium-sulfate-aluminate compounds (Concerte, J. F. Young, pp 76-98, Prentice-Hall, Inc, Englewood Cliffs, 1981). In permanent and long term implants, such as dental fillings, bone implants, and orthopedic surgery, the calcium sulfate aluminates will continually release aluminum ions into the human biological system (Fridland, et al., “MTA Solubility: A Long Term Study”, JOE—Volume 31, Number 5, May 2005, and Journal of Endodontics, Vol. 29, No. 12, December 2003). Considerable literature indicates that aluminum ions are toxic to human biological systems. For example, aluminum directly inhibits mineralization of bone or is toxic to the osteoblast. Diseases that have been associated with aluminum include dialysis dementia, renal osteodystrophy and Alzheimer's disease; aluminum also has an effect on red blood cells, parathyroid glands and chromosomes.
Recently, a white form of MTA (which is substantially iron-free) has been released, which addressed the concerns related to color-compatibility for dental applications. However, the modified “white” MTA is still essentially OPC with aluminum as one of its components. Primus (US Pat. Appl. No. 20030159618) disclosed a process for making a white, substantially non-iron containing dental material formed from Portland cement. The material still contains aluminum in its chemical composition. Moreover, while this process decreases the iron content it does not improve the biological properties of these materials, because it does not include any calcium phosphate phases and in particular does not include hydroxyapatite.
Hydraulic calcium phosphate cements (CPC) are another material widely used for variety of bio-medical applications. CPC was first reported in a binary system containing tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrate (DCPA) (L. C. Chow et al. J. Dent Res., 63, 200, 1984). The CPC advantages include self-setting (similar to OPC), but additionally it includes an apatitic phase in the set cement (e.g. HAP) Consequently, CPC is a bio-active material actively interacting with body fluids through dissolution-reprecipitation process. This has led to applications such as bone replacement and reconstruction, and also drug delivery devices (M. Dairra, et al. Biomaterials, 19 1523-1527, 1998; M. Otsuka, et al. J. of Controlled Release 43(1997)115-122, 1997; Y. Tabata, PSTT, Vol. 3, No. 3, 80-89, 2000; M. Otsuka, et al. J. of Pharm. Sci. Vol. 83, No. 5, 1994). Research into CPC is quite active, and two inventors in the present matter were also co-inventors in U.S. Pat. No. 6,730,324 which disclosed a novel process for CPC and its applications.
CPC is typically formulated as a mixture of solid and liquid components in pertinent proportions, which react to form the apatite HAP. The physicochemical reactions that occur upon mixing of the solid and liquid components are complex, but dissolution and precipitation are the mechanisms primarily responsible for the final apatite formation (C. Hamanish et al J. Biomed. Mat. Res., Vol. 32, 383-389, 1996; E. Ferandez et al J. Mater. Sci. Med. 10, 223-230, 1999). The reaction pathway in most CPC systems does not lead to stoichiometric HAP, but rather to calcium-deficient Ca10-x(HPO4)x(PO4)6-x(OH)2-x, similar to that found in bone. The major drawback of CPC technology is low mechanical strength (generally below 20 MPa compressive), which extremely limits its applications for medical materials and devices.
Silica also enhances the bioactive properties of materials. A combination of the oxides of calcium, phosphorous and silicon in proper proportions (with a majority of silica, of about 45 wt %) results in a well known bioactive glass material, with excellent in-vivo performance and stimulation of cell growth (e.g. Oonishi et al, “Particulate Bioglass compared with hydroxyapatite as a bone graft substitute”, J. Clin. Orthop. Rel. Res. 334, 316-25, 1997; also U.S. Pat. No. 5,811,302 by Ducheyne et al, Sep. 22, 1988). Unfortunately, although chemically advantageous, bio-glass must be processed at very high temperatures (generally in excess of 1000 C), and is rather a dense, weak and brittle material. Another disadvantage of bio-glass is that it does not easily dissolve in biological environment (due to dense SiO2 film coverage), which is desirable in some applications, e.g., for stimulation of bone growth.
The literature has reported recent attempts to address these issues, by combining the three oxides of calcium, phosphorous and silicon into porous crystalline composite material, which would possess high bioactivity similar to the bio-glass, but which would be stronger (even though porous) and easier to resorb in-vivo (A. R. El-Ghannam, “Advanced bioceramics composite for bone tissue engineering: design principles and structure-bioactivity relationship”, J. Biomed. Mater. Res. 69A, 490-501, 2004). The precursors to the three oxides (plus sodium oxide) were heat treated at high temperatures (130-800 C) to result in a porous composite of crystalline silica and variety of calcium-phosphates or calcium-sodium-phosphates. Excellent bioactivity of these composites was demonstrated. Unfortunately the need for the high temperature treatment makes this composite material difficult to use as biomaterial, as all the processing and shaping operations must take place outside of the application/implantation site.
Accordingly, there exists a need for a hydraulic cement having improved properties, in particular high early strengths, high overall compressive strength, rapid setting time, low hydration heat, resistance to degradation, and good expansiveness to offset shrinkage. Still further, there exists a need for such a hydraulic cement that can be readily modified to have biocompatible and bioactive properties, so as to be useful for medical and dental applications, as well as engineering applications.